Note: Descriptions are shown in the official language in which they were submitted.
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POWER GENERATION
The present invention relates to a method and an
apparatus for generating electrical power that is based on
the use of coal bed methane gas as a source of energy for
driving a gas turbine and a steam turbine for generating
the power.
The term "coal bed methane" is understood herein
to mean gas that contains at least 75% methane gas on a
volume basis obtained from an underground coal source.
According to the present invention there is
provided a method of generating power via a gas turbine
and a steam turbine which comprises:
(a) supplying coal bed methane, an oxygen-
containing gas, and flue gas produced in
the gas turbine, all under pressure, to a
combustor of the gas turbine and combusting
the coal bed methane and using the heated
combustion products and the flue gas to
drive the gas turbine;
(b) supplying a hot flue gas stream produced in
the gas turbine to a heat recovery steam
generator and using the heat of the flue
gas to generate steam by way of heat
exchange with water supplied to the steam
generator;
(c) suppling steam from the steam generator to
a steam turbine and using the steam to
drive the steam turbine; and
(d) supplying (i) a part of the flue gas stream
from the gas turbine that passes through
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the heat recovery steam generator to the
combustor of the gas turbine and (ii)
another part of the flue gas stream from
the gas turbine that passes through the
heat recovery steam generator to a suitable
underground storage region.
One of the features of the method and the
apparatus of the present invention is that ,it can operate
with no COZ emissions into the atmosphere.
By way of example, supplying all of the flue gas,
which inevitably contains substantial amounts of C02, that
is not supplied to the combustor of the gas turbine to the
suitable underground storage is an effective option for
preventing COz emissions into the atmosphere that does not
have any adverse environmental consequences.
Another feature of the present invention is that
the use of part of the flue gas stream from the gas
turbine in the combustor of the gas turbine makes it
possible to reduce, and preferably replace altogether, the
use of air in the combustor of the gas turbine. The total
replacement of air with oxygen and flue gas, which is
predominantly C02 in this mode of operation, overcomes
significant issues in relation to the use of air. For
example, the use of air means that the flue gas stream
from the gas turbine contains a significant amount
(typically at least 70 vol.~) nitrogen, an amount
(typically 10 vol.~) oxygen, and an amount (typically 5-10
vol.~) COZ. The mixture of nitrogen, oxygen, and C02
presents significant gas separation issues in order to
process the flue gas stream properly. The replacement of
air with oxygen and flue gas means that the flue gas
stream from the heat recovery steam generator is
predominantly COZ and water and greatly simplifies the
processing requirements for the flue gas from the gas
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turbine, with the result that it is a comparatively
straightforward exercise to produce a predominately COZ
flue gas stream and supply the stream to the combustor of
the gas turbine.
Preferably the oxygen-containing gas supplied to
the combustor of the gas turbine is oxygen.
Preferably the flue gas stream supplied to the
combustor of the gas turbine is predominantly COZ.
Preferably step (d) includes supplying part of
the flue gas stream to the combustor of the gas turbine
and the remainder of the flue gas stream to the
underground storage.
Preferably step (d) includes supplying the flue
gas stream to the underground storage region as a liquid
phase.
Preferably the underground storage region is a
coal bed seam.
More preferably the underground storage region is
the coal bed seam from which coal bed methane to power the
gas turbine is extracted. In this context, the existing
well structures for extracting coal bed methane can be
used to transfer flue gas, in liquid or gas phases, to the
underground storage region.
Preferably step (d) includes supplying the flue
gas stream to the underground storage region via existing
well structures for extracting coal bed methane from the
underground storage region.
Preferably step (d) includes separating water
from the flue gas.
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Step (d) may further include:
(i) compressing the flue gas stream to a
first pressure (typically 20-30 bar); and
(ii) supplying one part of the compressed flue
gas stream to the combustor of the gas
turbine.
Step (d) may further include:
(i) compressing another part of the
compressed flue gas stream to a second,
higher pressure (typically at least 70
bar, more typically at least 73 bar);
(ii) cooling the pressurised flue gas stream
from step (i) and forming a liquid phase;
and
(iii) supplying the liquid phase to the
underground storage region.
According to the present invention there is also
provided an apparatus for generating power via a gas
turbine and a steam turbine which comprises:
(a) a gas turbine;
(b) a means for supplying coal bed methane, an
oxygen-containing gas, and flue gas
produced in the gas turbine, all under
pressure, to a combustor of the gas turbine
for combusting the coal bed methane and
using the heated combustion products and
the flue gas to drive the gas turbine;
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(c) a heat recovery steam generator for
generating steam from water supplied to the
steam generator by way of heat exchange
with a flue gas from the gas turbine;
(d) a steam turbine adapted to be driven by
steam generated in the steam generator;
(e) a means for supplying (i) one part of a
flue gas stream from the gas turbine that
passes through the heat recovery steam
generator to the combustor of the gas
turbine and (ii) another part of the flue
gas stream from the gas turbine that passes
through the heat recovery steam generator
to a suitable underground storage region.
Preferably the means for supplying one part of
the flue gas stream to the combustor of the gas turbine
and another part of the flue gas stream to the suitable
underground storage region includes a means for converting
the flue gas from a gas phase into a liquid phase to be
supplied to the suitable underground storage region.
In a situation in which the oxygen-containing gas
for the combustor of the gas turbine includes oxygen,
preferably the apparatus further includes an oxygen plant
for producing oxygen.
The present invention is described further with
reference to the accompanying drawing which is one,
although not the only, embodiment of a power generation
method and apparatus of the invention.
With reference to the figure, the method includes
supplying the following gas streams to a combustor 5 of a
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gas turbine generally identified by the numeral 7:
(a) coal bed methane from an underground source
3 via a dedicated coal bed methane
compressor station (not shown) and a supply
line 51;
(b) oxygen, in an amount required for
stoichiometric combustion, produced in an
oxygen plant in the form of an air
separation plant 11, via a line 53;
(c) flue gas, which is predominantly C02, that
has been supplied from a flue gas stream
from the turbine 7, described hereinafter,
via a line 55.
The streams of oxygen and flue gas are pre-mixed
in a mixer 9 upstream of the combustor 5.
The streams of coal bed methane and oxygen/flue
gas are supplied to the combustor 5 at a preselected
pressure of between 16 and 28 bar. It is noted that the
combustor may operate with any suitable pressure.
The coal bed methane is combusted in the
combustor 5 and the products of combustion and the flue
gas are delivered to an expander 13 of the turbine 7 and
drive the turbine blades (not shown) located in the
expander 13.
The output of the turbine 7 is connected to and
drives an electrical generator 15 and a multiple stage
flue gas compressor train 17.
When the power generation method is operating in
this mode, air in the air compressor 21 of the turbine 7
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is bled at approximately 5 bar pressure and delivered to
the air separation plant and is used to produce oxygen for
the combustor 5 of the gas turbine 7.
The output gas stream, ie the flue gas, from the
turbine 7 is at atmospheric pressure and typically at a
temperature of the order of 540°C.
The flue gas from the turbine 7 is passed through
a heat recovery steam generator 27 and is used as a heat
source for producing high pressure steam, typically
approximately 75 bar or 7.5 Mpa, from demineralised water
and condensate return supplied to the steam generator 27.
The high pressure steam is supplied via a line 57
to a steam turbogenerator 29 and is used to run the
turbogenerator 29 and generate electrical power.
The flue gas from the heat recovery steam
generator 27, which is predominantly COz and water, leaves
the steam generator as a wet flue gas stream, typically at
a temperature of 125°C, via an outlet 19.
The wet flue gas is then passed through a water
separator 33 that separates water from the stream and
produces a dry flue gas stream.
The dry flue gas stream is then passed through
the multiple stage flue gas compressor train 17.
In a first stage of compression the flue gas is
compressed to the necessary pressure, namely 22 bar in the
present instance, for the combustor 5 of the turbine 7.
Compressed flue gas from the exit of the first
stage is supplied to the combustor 5 of the turbine 7 via
the mixer 9, typically a mix valve, and mixes with oxygen
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from the air separator 11 prior to being supplied to the
combustor 5.
The remainder of the flue gas is supplied to the
second compression stage, marked "Stage 2" in the figure,
and is compressed to a high pressure, typically above 73
bar, and the stream of compressed flue gas is then passed
through a condenser 35. The condenser 35 cools the
temperature of the flue gas stream to below 31°C and
thereby converts the flue gas to a liquid phase.
The liquid flue gas stream leaving the condenser
is pressurised (if necessary) arid then injected into
existing field wells.
When the power generation system is not operating
in the above-described mode and, more particularly is not
receiving the stream of pre-mixed oxygen and flue gas, the
turbine 7 operates on a conventional basis with air being
drawn through the turbine air intake (not shown) and
compressed in the air compressor 21 and thereafter
delivered to the combustor 5 and mixed with coal bed
methane and the mixture combusted in the combustor 5.
More particularly, the option of operating on a
more conventional basis is available by disconnecting the
multiple stage flue gas compressor train 17 from the
turbine 7.
The key components of the above-described
embodiment of the process and the apparatus of the
invention shown in the figure are as follows:
(a) Air Separation Plant - This unit is
required to produce oxygen for combustion
of coal bed methane in the turbine
combustor. Typically, the plant is a
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standard off-the-shelf unit sized to cope
with the Oz required for complete
combustion of coal bed methane.
(b) Gas Turbine/Generator - Typically, this
unit is a standard gas turbine fitted with
a standard combustor. The multi-stage flue
gas compressor will be fitted on the same
shaft with a clutch unit that will enable
the compressor to be isolated when the
turbine is operating in a conventional
manner. The attachment of large multi-stage
compressors to gas turbine units is quite
common in the steel industry where low Btu
steelworks gases are compressed by these
units before being delivered to the
combustor for combustion.
(c) Heat Recovery Steam Generator - Typically,
this unit is a standard double pressure
unfired unit.
(d) Steam Turbine/Generator - Typically, this
unit, complete with the steam cycle
ancillaries, is a standard steam turbine
unit.
(e) Flue Gas Recirculating and C02 Underground
storage System - Typically, this system
contains the following:
(i) Water Separator/knockout Unit -
Typically this unit is a simple
water separation plant in which
water is knocked out of the flue
gas stream prior it entering the
multi-stage compressor unit.
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(ii) C02 multi-stage compressor train -
For the embodiment shown in Figure
1, typically this unit is designed
to handle the entire flue gas
stream in the first stage of
compression and the smaller stream
of flue gas for underground storage.
This smaller stream will be
pressurised to above 73 bar before
being delivered to the condenser.
(iii) Condenser - This unit is required
to produce liquid flue gas, which
is predominantly COZ, prior to
injecting it to underground wells.
Many modifications may be made to the embodiments
of the present invention described above without departing
from the spirit and scope of the invention.
By way of example, in another, although not the
only other possible, embodiment of the method and the
apparatus of the invention, the flue gas from the steam
generator 27 is passed through a recuperator and is cooled
to a temperature, typically 80C, before being transferred
to the water separator 33. In addition, the dry flue gas
is not split into two streams after the first stage in the
multiple stage flue gas compressor train 17, as is the
case in the embodiment shown in the figure. Rather, the
whole of the dry flue gas from the water separator 33 is
compressed in the compressor train 17 and then passed
through the condenser 35. The liquid stream from the
condenser 35 is then split into two streams, with one
stream being supplied to the underground storage region
and the other stream being passed through the recuperator
31 and being converted into a gas phase via heat exchange
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with the flue gas stream from the steam generator 27. The
reformed flue gas from the recuperator 31 is then supplied
to the combustor 5 via the mixer 9.
